Establishing robust chronologies for two young stalagmites from the tropical south Pacific using radiocarbon
Mr Mohammadali Faraji1, Dr Quan Hua2, Dr Andrea Borsato1, Prof Silvia Frisia1
1The University Of Newcstle, Australia, 2ANSTO, Australia
In the last couple of decades, speleothems have provided terrestrial paleoclimate reconstructions covering almost all latitudes and a with temporal resolutions varying from seasonal to millennial scales. The most important property of speleothems is their capability to be dated via radiometric methods. The most commonly used dating method is based on U-series disequilibrium. When this method is not applicable, for example, because of multiple sources of 230Th, robust chronologies can be acquired via integrating the counting of annual physical and chemical laminae in two-dimensional maps. This approach may also entail uncertainties related to the exact age of the youngest parts of the speleothem, commonly assumed to be actively growing at the time of removal. An alternative/complementary method for obtaining accurate age models and/or further constrain the age of speleothems that grew in the last circa 100 years is using the radiocarbon “bomb-pulse”. However, when comparing the radiocarbon age of speleothems with other contemporaneous terrestrial samples which absorb carbon from atmospheric CO2, speleothems show older than expected ages. That is due to the contribution of ‘radioactively dead’ carbon, known as Dead Carbon Fraction (DCF), derived from 14C-depleted material from bedrock and aged soil organic matter. Yet, a thorough understanding of DCF variations in speleothems potentially generates reliable chronologies.
Relatively young Tropical Pacific stalagmites are commonly difficult to date with the U-series, and radiocarbon is, potentially, the best dating method. We studied stalagmites Pu17 and Nu16, retrieved from Pouatea and Nurau caves in Atiu, the second largest island from the southern group of the Cook Islands in the South Pacific. Both were studied with the aim to construct robust radiocarbon chronologies and attain information about the hydrogeology of the caves. These caves are expected to provide an excellent opportunity for radiocarbon dating because the rock burden above caves is thin (4 to 8 m), characterized by high porosity, and a limited and patchy soil cover is. This ensures rapid transmission of surface climate parameters into the cave. Rapid transmission of rain signal into the cave reduces the interaction between rainwater and bedrock, thus, minimizing the contribution of bedrock-derived dead carbon, which is crucial for constructing accurate and robust chronologies.
Forty samples from the top portions of Pu17 and Nu16 were analysed for 14C using the VEGA AMS Facility at ANSTO. By modelling the soil carbon pools, we evaluated the age distribution of soil carbon above the cave. Results indicate a young epikarst carbon pool (younger than 22 years) for both Pu17 and Nu16, which translates into less than 4% DCF contribution. Radiocarbon chronologies of the two stalagmites show a sharp and relatively early bomb peak at ca. 1970 CE. The radiocarbon bomb-pulse allows to anchor chemical lamina counting and obtain accurate chronologies for modern speleothem and thus obtain robust paleorainfall records for the climate-vulnerable south Pacific Island communities.
I am Mohammadali, a last-year PhD candidate at the University of Newcastle. My research project aims to increase knowledge about hydroclimate variability in the South Pacific by providing a reconstruction of at least 350 years of rainfall variability from stalagmites from Atiu and interpreting such variability in terms of SPCZ (South Pacific Convergence Zone) shifts. This will help to improve climate projections and devise adaptation strategies in the climate-vulnerable South Pacific island communities.
I hold a bachelor’s degree and a master’s degree in Geosciences. I have found myself interested in Carbonate sedimentology, Sequence stratigraphy, and Paleoclimate reconstructions from geologic archives.